A hydraulic working machine is equipped with a hydraulic circuit that includes hydraulic actuators such as a hydraulic motor and hydraulic cylinders, hydraulic pumps, spool valves or the like, and hydraulic equipment drivable by hydraulic oil. Solid foreign matter is mixed in the hydraulic oil that flows through the hydraulic circuit. There are various kinds of foreign matter including: foreign matter that remains in the hydraulic equipment, piping and a hydraulic oil tank after assembly of the hydraulic circuit (chips from metal working, foundry sand, dust, and the like), foreign matter that is produced inside the hydraulic circuit during operation of the hydraulic circuit (abrasion powder occurring from sliding parts of the hydraulic equipment, sludge occurring as a result of oxidation or degradation of the hydraulic oil, and the like), foreign matter that enters from an external environment into the hydraulic system during operation of the hydraulic circuit, and foreign matter that enters the hydraulic circuit upon maintenance or repair. In particular, foreign matter of substantially the same particle sizes (5 to 50 μm) as the dimensions of clearances in the sliding parts of the hydraulic equipment causes abnormal wear or seizure of the sliding parts and induces an operation failure of the hydraulic working machine.
From such circumstances, it is necessary to perform maintenance of hydraulic oil, such as flushing or replacement, before an operation failure occurs on a hydraulic working machine due to its contamination with foreign matter. For determining whether or not it is a time to perform maintenance, it has conventionally been conducted to determine the level of contamination of hydraulic oil with foreign matter by using a contamination level determination system for hydraulic oil.
Conventional contamination level determination systems are each provided with a light-blocking sensor for sensing foreign matter in hydraulic oil. This light-blocking sensor has a light-emitting section for irradiating laser light onto the hydraulic oil flowing through a line and a light-receiving section for receiving the laser light. These conventional contamination level determination systems are designed to determine the contamination level of hydraulic oil on the basis of a change in the luminous intensity of laser light received at the light-receiving section.
Contamination level determination systems for hydraulic oil, each of which makes use of a light-blocking sensor as described above, involve a potential problem that, when plural pieces of foreign matter exist in laser light at the same time, these plural pieces of foreign matter may be miss-sensed as a large lump of foreign matter. This miss-sensing leads to a reduction in the accuracy of determination of the contamination level of hydraulic oil contaminated with foreign matter of particle sizes that may cause an operation failure.
Contamination level determination systems for hydraulic oil, which are of the above-mentioned type, include those improved in the sensing accuracy of particle sizes of foreign matter by a light-blocking detector of complex construction (see, for example, Patent Document 1).    Patent Document 1: JP-B-3890036